/*
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* (C) Copyright 2008
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* Gary Jennejohn, DENX Software Engineering GmbH <garyj@denx.de>
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*
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* SPDX-License-Identifier: GPL-2.0+
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*/
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U-Boot console multiplexing
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===========================
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HOW CONSOLE MULTIPLEXING WORKS
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------------------------------
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This functionality is controlled with CONFIG_CONSOLE_MUX in the board
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configuration file.
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Two new files, common/iomux.c and include/iomux.h, contain the heart
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(iomux_doenv()) of the environment setting implementation.
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iomux_doenv() is called in common/cmd_nvedit.c to handle setenv and in
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common/console.c in console_init_r() during bootup to initialize
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stdio_devices[].
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A user can use a comma-separated list of devices to set stdin, stdout
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and stderr. For example: "setenv stdin serial,nc". NOTE: No spaces
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are allowed around the comma(s)!
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The length of the list is limited by malloc(), since the array used
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is allocated and freed dynamically.
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It should be possible to specify any device which console_assign()
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finds acceptable, but the code has only been tested with serial and
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nc.
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iomux_doenv() prevents multiple use of the same device, e.g. "setenv
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stdin nc,nc,serial" will discard the second nc. iomux_doenv() is
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not able to modify the environment, however, so that "pri stdin" still
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shows "nc,nc,serial".
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The major change in common/console.c was to modify fgetc() to call
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the iomux_tstc() routine in a for-loop. iomux_tstc() in turn calls
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the tstc() routine for every registered device, but exits immediately
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when one of them returns true. fgetc() then calls iomux_getc(),
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which calls the corresponding getc() routine. fgetc() hangs in
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the for-loop until iomux_tstc() returns true and the input can be
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retrieved.
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Thus, a user can type into any device registered for stdin. No effort
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has been made to demulitplex simultaneous input from multiple stdin
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devices.
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fputc() and fputs() have been modified to call iomux_putc() and
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iomux_puts() respectively, which call the corresponding output
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routines for every registered device.
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Thus, a user can see the ouput for any device registered for stdout
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or stderr on all devices registered for stdout or stderr. As an
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example, if stdin=serial,nc and stdout=serial,nc then all output
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for serial, e.g. echos of input on serial, will appear on serial and nc.
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Just as with the old console code, this statement is still true:
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If not defined in the environment, the first input device is assigned
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to the 'stdin' file, the first output one to 'stdout' and 'stderr'.
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If CONFIG_SYS_CONSOLE_IS_IN_ENV is defined then multiple input/output
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devices can be set at boot time if defined in the environment.
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CAVEATS
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-------
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Note that common/iomux.c calls console_assign() for every registered
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device as it is discovered. This means that the environment settings
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for application consoles will be set to the last device in the list.
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On a slow machine, such as MPC852T clocked at 66MHz, the overhead associated
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with calling tstc() and then getc() means that copy&paste will normally not
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work, even when stdin=stdout=stderr=serial.
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On a faster machine, such as a sequoia, cut&paste of longer (about 80
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characters) lines works fine when serial is the only device used.
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Using nc as a stdin device results in even more overhead because nc_tstc()
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is quite slow. Even on a sequoia cut&paste does not work on the serial
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interface when nc is added to stdin, although there is no character loss using
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the ethernet interface for input. In this test case stdin=serial,nc and
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stdout=serial.
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In addition, the overhead associated with sending to two devices, when one of
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them is nc, also causes problems. Even on a sequoia cut&paste does not work
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on the serial interface (stdin=serial) when nc is added to stdout (stdout=
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serial,nc).
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